1. Field of the Invention
The present invention relates to an image display device having a memory property and more particularly to the image display device having the memory property that can be suitably used for electronic paper display device such as electronic books, electronic newspaper, and the like.
2. Description of the Related Art
As a display device capable of doing a deed of “reading” without stress, an electronic paper display device referred to as an electronic book, electronic newspaper and the like is now under development. Since it is necessary that the electronic paper display of this kind is thin, lightweight, hard to crack, and low in power consumption, its construction by using a display element having a memory property is desirable. As a display element to be used in a device having a memory property, conventionally, an electrophoretic display element or cholesteric. liquid crystal or the like is known, however, in recent years, electrophoretic display elements of two or more kinds are attracting attention. In this specification, the electrophoretic display element conceptually contains a device such as a powder element that can achieve displaying by causing electrically charged particles to move in a solvent or the like.
Hereinafter, an electrophoretic display device of a type that displays white and black colors by active-matrix driving method is described. The electrophoretic display device is so configured that a TFT (Thin-Film Transistor) glass substrate, electrophoretic display element film, and facing substrate are stacked in layers in this order. On the TFT glass substrate, TFTs serving as a plurality of switching elements arranged in a matrix manner, pixel electrodes, gate lines, and data lines are mounted. The electrophoretic display device is constructed in a manner in which micro capsules being about 40 μm in size spread in a polymer binder. A solvent is injected into an inner portion of each of the microcapsules and, in the solvent, two kinds of positively and negatively-charged nano-particles, that is, a white pigment made up of negatively charged titanium dioxide particles and a black pigment made up of positively charged carbon particles are confined within a dispersed and floated state. Moreover, on the facing substrate, a facing electrode (common electrode) to provide a reference potential is formed.
The electrophoretic display device is operated by applying a voltage corresponding to pixel data between the pixel electrode and facing electrode and by moving the white and black pigments up and down. That is, when a positive voltage is applied to the pixel electrode, the negatively charged white pigment is attracted by the pixel electrode while the positively charged black pigment is attracted by the facing electrode and, therefore, by using the facing electrode side as its display surface, black is displayed on the screen. Further, when a negative voltage is applied to the pixel electrode, the positively charged black pigments are attracted by the pixel electrode while the negatively charged white pigments are attracted by the facing electrode and, as a result, white is displayed on the screen. Next, when an image display is to be changed from white to black, a positive signal voltage is applied to the pixel electrode and, when the image display is changed from black to white, a negative signal voltage is applied to the pixel electrode, and when a current image display is maintained, that is, the image display is changed from white or white and from black to black, 0V is applied. Thus, sine the electrophoretic display element has a memory property, by comparing the current image (previous image) and next image (renewed image), a signal to be applied is determined.
In the above, the white and black display microcapsule type electrophoretic display device is described. However, an advent of an electrophoretic display device that can display colors without losing an excellent display state in white and black as in the case of paper and without using a color filter is further expected and an electrophoretic display device that can display bright color even in order of unit pixel is still under development.
For example, in Patent Reference 1 (Japanese Patent No. 4049202), an electrophoretic display device is disclosed which includes a pair of substrates, a dispersion medium enclosed between the pair of substrates, an electrophoretic particle contained in the dispersion medium having either positive or negative charge of the same polarity and providing three colors each being different from one another, (for example, cyan (C), magenta (M), and yellow (Y)), and a white (W) support body to support the electrophoretic particles. In the electrophoretic display device disclosed in the Patent Reference 1, by setting a voltage (hereinafter “threshold value voltage”) to initiate the movement of the electrophoretic particle having three colors each being different from one another and by applying a voltage by using a difference in threshold voltage (absolute value), one cell can display cyan (C), magenta (M), and yellow (Y) in addition to white (W) and black (K), and second color and third color of these CMY colors.
Moreover, in Patent Reference 2 (Japanese Patent No. 4385438), a color electrophoretic display device is also disclosed which includes a black particle having charge of a first polarity, a particle (electrophoretic particle) having charge of second polarity opposite to the first polarity, a liquid dispersion medium to disperse these particles in a manner in which electrophoresis can occur, and an electrophoretic display element film in which a plurality of kinds of microcapsules with these media enclosed therein is stacked in layers. In the microcapsule disclosed in the Patent Reference 2, particles having charge of the second polarity of red (R), green (G), and blue (B) each being different in charged amount are enclosed for every kind of the microcapsule.
By utilizing the fact that the charged amount of each of the particles (R), (G), and (B) having charge of the second polarity is different from one another and that the threshold voltage of red (R), green (G), and blue (B) having charge of second polarity is also different in color from one another and, as in the case of the Patent Reference 1, bright color display has been realized without using a color filter.
Further, in Patent Reference 3 (Japanese Patent Application Laid-open No. 2009-47737), a color electrophoretic display element is disclosed which uses electrophoretic particles having not only 3 colors including cyan (C), magenta (M), and yellow (Y) but also a color of black (K), 4 colors in total.
In brief, the display devices disclosed in the Patent References 1, 2, and 3 show that color displaying can be achieved by allowing the charged particles C, M, and Y (or charged particles R, G, and B) to have three threshold value voltages each being different from one another. However, when color displaying of three charged particles C, M, and Y is to be performed by using a difference in threshold value at a same pixel electrode, the driving for realizing a targeted color displaying is actually very complicated.
This problem is described by paying attention to a behavior of the electrophoretic particle disclosed in the Patent Reference 1 using FIGS. 27 and 28. It is hereinafter assumed that, when the threshold value voltage of each of the electrophoretic particles (charged particle) C, M and Y is Vth(c), Vth(m), and Vth(y), respectively, the characteristic relationship of |Vth(c)|<|Vth(m)|<|Vth(y)| (absolute value) is established. Moreover, applied voltages V1, V2, and V3 satisfy the characteristic relationship of |Vth(c)|<|V3|<|Vth(m)|, |Vth(m)|<|V2|<|Vth(y)|, and |Vth(y)|<|V1|. FIGS. 27 and 28 show hysteresis curves of the electophoretic particles C, M, and Y which represents the characteristic relationship between a threshold voltage and a relative color density. In these drawings, for simplification of the descriptions, in order to set the tilt of each of the hysteresis curves Y, nY, M, nM, C, and nC to be constant, the time when the Y, M, and C move from a rear surface to a display surface is set to different time.
As shown in FIG. 27, it is assumed that the image at the point of time of starting (previous image) is first set to white (W). When the voltage V3 (=10V) is applied, an electrophoretic particle having a cyan color moves to the display surface side, resulting in display of cyan (C) and, when the voltage V2 (=15V) is applied, an electrophoretic particle having a cyan and a magenta color moves to the display surface side, resulting in display of a blue (B) color. Also, when the voltage V1 (=30V) is applied, the electrophoretic particle having the cyan color, the electrophoretic particle having the magenta color, and the electrophoretic particle having the yellow color move to the display surface side, resulting in display of black (K). Moreover, when a previous image has been set to white (W), if a negative voltage is applied, no color particles move to the display surface side and, therefore, the image remains white (W).
Next, when the previous image has been set to black (K) and, if the negative voltage of −V3 (=−10V) is applied, an electrophoretic particle having the cyan color moves to a rear surface substrate side and the electrophoretic particle having the magenta (M) color and electrophoretic particle having the yellow (Y) color are left on the display surface side, thus resulting in display of red (R). When the previous image has been set to black (K), if the voltage of −V2 (=−15V) is applied, the electrophoretic particle having the cyan and magenta colors move to the rear substrate side and only the electrophoretic particle having the yellow (Y) color is left on the display surface side, thus resulting in display of a yellow (Y) color. When the previous image has been set to black (K), if the voltage of −V (=−30V) is applied, the electrophoretic particle having all the colors of cyan (C), magenta (M), and yellow (Y) move to the rear substrate side, thus resulting in display of white (W).
Next, for displaying a green (G) color or magenta (M) color, a driving method being different from the display method applied to driving of red (R), blue (B), cyan (C), yellow (Y), white (W) and black (K) are employed. For example, in order to display magenta (M), as shown in FIG. 28, the voltage of V2 (=15V) is applied to an image for displaying a white (W) color to once change a display color to blue (B). Therefore, by applying the voltage of −V3 (=−10V) to move the electrophoretic particle having a cyan color, a magenta color is then displayed.
Thus, the driving methods for displaying primary colors of red (R), green (G), blue (B), cyan (C), magenta (M), yellow (Y), white (W), and black (K) are described, however, the related driving method for displaying a given color La*b* including an intermediate color and shades of gray is very complicated. The above discussion holds true for the color micro capsule type electrophoretic display device disclosed in the Patent Reference 2 and/or the electrophoretic display device for displaying 4 colors of C, M, Y, and K.